1. Field of the Invention
This invention relates to computer system input/output (I/O) nodes and, more particularly, to transaction handling in a tunnel device for an I/O node.
2. Description of the Related Art
In a typical computer system, one or more processors may communicate with input/output (I/O) devices over one or more buses. In addition those processors may communicate with each other through an additional bus or buses. In many cases, these buses are shared buses.
Unfortunately, many shared bus systems suffer from drawbacks. For example, multiple devices attached to a bus may present a relatively large electrical capacitance to devices driving signals on the bus. In addition, the multiple attach points on a shared bus produce signal reflections at high signal frequencies which reduce signal integrity. As a result, signal frequencies on the bus are generally kept relatively low in order to maintain signal integrity at an acceptable level. The relatively low signal frequencies reduce signal bandwidth, limiting the performance of devices attached to the bus. An example of a shared bus used by many systems is a front side bus (FSB), which may typically interconnect one or more processors and a system controller.
To overcome some of the drawbacks of a shared bus, some computers systems may use packet-based communications such as point-to-point links, for example, between devices or nodes. In such systems, nodes may communicate with each other by exchanging packets of information. In general, a “node” is a device which is capable of participating in transactions upon an interconnect. For example, the interconnect may be packet-based, and the node may be configured to receive and transmit packets. Generally speaking, a “packet” is a communication between two nodes: an initiating or “source” node which transmits the packet and a destination or “target” node which receives the packet. When a packet reaches the target node, the target node accepts the information conveyed by the packet and processes the information internally. A node located on a communication path between the source and target nodes may relay or forward the packet from the source node to the target node.
When some of these nodes forward a transaction, the transaction may be received into the forwarding node at one frequency and on a bus having a given width. The transaction may then be internally forwarded onto another bus or path having a different frequency and width before the transaction is finally forwarded onto a second external bus.